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4.2 With ADS-B (ADS-Broadcast) the ADS transmissions are sent regularly by the aircraft equipment with no external initiation. Recipients may be ground stations or other suitably equipped aircraft (with ADS-B IN capability). ADS-B IN allows for other applications such as displaying in the cockpit traffic information (Traffic Information Service TIS-B), Met, Notam and ATIS information (Flight Information Service FIS-B) and for the Airborne Separation Assistance System (ASAS). For full situational awareness the TIS-B information uplinked to aircraft may have been originally sourced from a wide range of sensors including multilateration and conventional surveillance as well as the ADS transmissions of other aircraft.
4.3 ADS-C (ADS-Contract) relies on a contract between an aircraft and a ground station such that the aircraft will automatically provide information obtained from its on-board sensors to the ground system under specific circumstances dictated by the ground system. Initiating circumstances include change of altitude, exceeding lateral navigation limits or the aircraft passing waypoints.
5 Multi-lateration (MLAT)
5.1 Uses Time Difference of Arrival (TDOA) of the same aircraft transmission at several sensors to enable calculation of aircraft position. Aircraft transmissions are usually responses to Mode-S interrogations or extended squitter. MLAT is usually used in a local area such as an airport to monitor ground movement, potentially aircraft and vehicles, but may be used in a wider area to provide a service equivalent to SSR. Accuracy of MLAT is dependant on many things including the geometry of the ground sensors, synchronisation of the sensors and a clean multipath environment. MLAT is usually a passive system relying on transmissions in response to routine mode-s interrogations or uninitiated 'squitter' transmissions although a dedicated interrogator may be co-located with the system to prompt responses ensuring sufficient transmissions are available.
6 Primary Surveillance Radar (PSR)
6.1 An independent surveillance method in which the aircraft has no co-operative role. The range to the target is determined by the round trip time of the radar pulse and the azimuth by the orientation of the rotating aerial at the time. Aircraft are detected purely on the radiation they reflect back to the receiver; as such the amount of energy received at the radar is related to the size, shape and composition of the target aircraft, known as the radar cross section (RCS). The potential range of a primary radar for a target of given RCS is dependant on the frequency and power output of the transmitter, with a maximum range of detection for a large aircraft is of the order of 200 NM at 25,000 feet.
7 Secondary Surveillance Radar (SSR)
7.1 A co-operative system in which the ground based equipment sends a coded transmission to which all suitably equipped aircraft that receive it will respond. The response is also a coded message that can convey such information as a selected identifier (squawk) (Mode 3A) and barometric altitude (Mode 3C) improving safety by enabling the positive identification of a target as well as its altitude. The transmitter power of the SSR ground station can be much less than a primary transmitter for similar ranges as the signal returned to the ground station is from an aircraft transmitter rather than reflected energy. As a result the ground equipment can be smaller and less expensive than would otherwise be the case. All ground based interrogators transmit on the same frequency and elicit replies from all airborne transponders on another fixed frequency, this coupled with the limited number of discrete aircraft identifiers (4 Octal digits) leads to congestion of the system making it inoperable as traffic levels rise.
8 Mode S SSR
8.1 Whilst traditional Secondary Surveillance Radar (SSR) stations interrogate all aircraft within their range, Mode S establishes selective and addressed interrogations with aircraft within its coverage. Such selective interrogation improves the quality and integrity of the detection, identification and altitude reporting. These improvements translate into benefits in terms of safety, capacity and efficiency; benefits which are key to supporting the future of the high traffic density airspace of Europe. This first step of selective interrogation is known as Mode S Elementary Surveillance. Mode S Enhanced Surveillance builds upon the concept of Elementary Surveillance and consists of the extraction of further aircraft parameters, facilitating an increase in the safety and efficiency of the ATM operations. Mode-S Extended Squitter can be used as the datalink service for ADS.
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CAP 670 Air Traffic Services Safety Requirements 2(79)
